Not true. If you can fit a PCI-Express card into a PCI-Express slot, it'll work. You can even put an x1 sound card into an x16 slot.

However, fender, you're going to likely have poor performance. The bandwidth on an x4 lane is a fraction of an x16 lane; even x8 is way more capable than an x4 lane. Many motherboards, if the primary x16 slot isn't in use, will reassign that power to a slot that IS in use. In other words, your motherboard might very well give x8 or x16 power to your x4 slot if your x16 one's empty. Refer to your motherboard manual to see for sure.